Why do the CH-47 Chinook helicopters have counter rotating rotor blades?

Most helicopters have a tail boom and tail rotor. This tail rotor is present to counteract the torque produced by a helicopters main rotor. If there was no tail rotor, the helicopter would tend to rotate in a direction opposite to that which the main rotor blade spins which would make flying difficult or even impossible.

The Chinook doesn't have a tail rotor, instead it has twin main rotors. If they both span in the same direction then this would act to cause the helicopter to twist. However as they rotate in opposite directions the two twisting forces cancel each other out. This has other advantages as it means that the full power of the engines goes towards generating lift and thrust as opposed to driving the vertical stabilising rotor on other helicopters.